Agonist refers to a drug that effectively mimics the action of a natural chemical messenger within the body.
An agonist is a substance that binds to a receptor and triggers a response in the cell. An agonist is the opposite of an antagonist in the sense that while an antagonist also binds to the receptor, it fails to activate the receptor and actually blocks it from activation by agonists. A partial agonist activates a receptor but does not cause as much of a physiological change as does a full agonist. The receptors of the human body work by being stimulated or inhibited by natural (such as hormones and neurotransmitters) or synthetic (such as drugs) agonists and antagonists.
Agonist refers to a muscle or muscle group that is described as being primarily responsible for a specific joint movement when contracting.